p0f v2 is a passive operating system fingerprinting tool. It can identify the OS on: - machines that connect to your box (SYN mode), - machines you connect to (SYN+ACK mode), - machines you cannot connect to (RST+ mode), - machines whose communications you can observe. P0f can also detect or measure the following: - firewall presence, NAT use, - existence of a load balancer setup, - the distance to the remote system and its uptime, - other guy's network hookup (DSL, OC3, avian carriers) and his ISP.